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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET-DRAFT Donald Eastlake 2 Intended Status: Proposed Standard Huawei 3 Mark Andrews 4 ISC 5 Expires: August 21, 2015 February 22, 2015 7 Domain Name System (DNS) Cookies 8 10 Abstract 12 DNS cookies are a lightweight DNS transaction security mechanism that 13 provides limited protection to DNS servers and clients against a 14 variety of increasingly common denial-of-service and amplification / 15 forgery or cache poisoning attacks by off-path attackers. DNS Cookies 16 are tolerant of NAT, NAT-PT, and anycast and can be incrementally 17 deployed. 19 Status of This Document 21 This Internet-Draft is submitted to IETF in full conformance with the 22 provisions of BCP 78 and BCP 79. 24 Distribution of this document is unlimited. Comments should be sent 25 to the author or the DNSEXT mailing list . 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF), its areas, and its working groups. Note that 29 other groups may also distribute working documents as Internet- 30 Drafts. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 The list of current Internet-Drafts can be accessed at 38 http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft 39 Shadow Directories can be accessed at 40 http://www.ietf.org/shadow.html. 42 Table of Contents 44 1. Introduction............................................4 45 1.1 Contents of This Document..............................4 46 1.2 Definitions............................................5 48 2. Threats Considered......................................6 49 2.1 Denial-of-Service Attacks..............................6 50 2.1.1 DNS Amplification Attacks............................6 51 2.1.2 DNS Server Denial-of-Service.........................6 52 2.2 Cache Poisoning and Answer Forgery Attacks.............7 54 3. Comments on Existing DNS Security.......................8 55 3.1 Existing DNS Data Security.............................8 56 3.2 DNS Message/Transaction Security.......................8 57 3.3 Conclusions on Existing DNS Security...................8 59 4. The COOKIE OPT Option...................................9 60 4.1 Client Cookie.........................................10 61 4.2 Server Cookie.........................................10 62 4.3 Error Code............................................11 64 5. DNS Cookies Protocol Description.......................12 65 5.1 Originating Requests..................................12 66 5.2 Responding to Requests................................12 67 5.2.1 No OPT RR...........................................13 68 5.2.2 No Valid Client Cookie..............................13 69 5.2.3 Bad or Absent Server Cookie.........................14 70 5.2.4 A Correct Server Cookie.............................14 71 5.3 Processing Responses..................................15 72 5.4 Client and Server Secret Rollover.....................15 73 5.5 Implementation Requirement............................16 75 6. Simple DNS Cookie Option...............................17 76 6.1 Simple Client Cookie..................................18 77 6.2 Simple Server Cookie..................................18 79 7. Simple DNS Cookies Protocol Description................20 80 7.1 Originating Requests (Simple).........................20 81 7.2 Responding to Request (Simple)........................20 82 7.2.1 No Opt RR or No COOKIE OPT option...................20 83 7.2.2 Malformed COOKIE OPT option.........................21 84 7.2.3 Only a CLIENT Cookie................................21 85 7.2.4 A Client Cookie and Server Cookie...................21 86 7.2.4.1 A Client Cookie and Invalid Server Cookie.........21 87 7.2.4.2 A Client Cookie and Valid Server Cookie...........21 89 8. NAT Considerations and AnyCast Server Considerations...23 90 9. Deployment.............................................25 92 Table of Contents (continued) 94 10. IANA Considerations...................................26 96 11. Security Considerations...............................27 97 11.1 Cookie Algorithm Considerations......................27 99 Normative References......................................28 100 Informative References....................................28 102 Acknowledgements..........................................30 104 Appendix A: Example Client Cookie Algorithms..............31 105 A.1 A Simple Algorithm....................................31 106 A.2 A More Complex Algorithm..............................31 108 Appendix B: Example Server Cookie Algorithms..............32 109 B.1 A Simple Algorithm....................................32 110 B.2 A More Complex Algorithm..............................32 112 Author's Address..........................................34 114 1. Introduction 116 As with many core Internet protocols, the Domain Name System (DNS) 117 was originally designed at a time when the Internet had only a small 118 pool of trusted users. As the Internet has grown exponentially to a 119 global information utility, the DNS has increasingly been subject to 120 abuse. 122 This document describes DNS cookies, a lightweight DNS transaction 123 security mechanism specified as an OPT [RFC6891] option. Two 124 different cookie formats are presented for evaluation. 126 The DNS cookies mechanism provides limited protection to DNS servers 127 and clients against a variety of increasingly common abuses by off- 128 path attackers. It is compatible with and can be used in conjunction 129 with other DNS transaction forgery resistance measures such as those 130 in [RFC5452]. 132 The protection provided by DNS cookies bears some resemblance to that 133 provided by using TCP for DNS transactions. To bypass the weak 134 protection provided by using TCP requires an off-path attacker 135 guessing the 32-bit TCP sequence number in use. To bypass the weak 136 protection provided by DNS Cookies requires such an attacker to guess 137 a 64-bit pseudo-random quantity. Where DNS Cookies are not available 138 but TCP is, a fall back to using TCP is a reasonable strategy. 140 If only one party to a DNS transaction supports DNS cookies, the 141 mechanism does not provide a benefit or significantly interfere; but, 142 if both support it, the additional security provided is automatically 143 available. 145 The DNS cookies mechanism is designed to work in the presence of NAT 146 and NAT-PT boxes and guidance is provided herein on supporting the 147 DNS cookies mechanism in anycast servers. 149 1.1 Contents of This Document 151 In Section 2, we discuss the threats against which the DNS cookie 152 mechanism provides some protection. 154 Section 3 describes existing DNS security mechanisms and why they are 155 not adequate substitutes for DNS cookies. 157 Section 4 describes the COOKIE OPT option including an error code 158 field. 160 Section 5 provides a protocol description including an error code 161 field. 163 Section 6 describes an alternative COOKIE OPT option not including an 164 error field. 166 Section 7 provides a description of a simplified protocol without an 167 error code. 169 Section 8 discusses some NAT and anycast related DNS Cookies design 170 considerations. 172 Section 9 discusses incremental deployment considerations. 174 Sections 10 and 11 describe IANA and Security Considerations. 176 1.2 Definitions 178 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 179 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 180 document are to be interpreted as described in [RFC2119]. 182 An "off-path attacker", for a particular DNS client and server, is 183 defined as an attacker who cannot observe the DNS request and 184 response messages between that client and server. 186 "Soft state" indicates information learned or derived by a host which 187 may be discarded when indicated by the policies of that host but can 188 be later re-instantiated if needed. For example, it could be 189 discarded after a period of time or when storage for caching such 190 data becomes full. If operations requiring that soft state continue 191 after it has been discarded, it will be automatically re-generated, 192 albeit at some cost. 194 "Silently discarded" indicates that there are no DNS protocol message 195 consequences; however, it is RECOMMENDED that appropriate network 196 management facilities be included in implementations, such as a 197 counter of the occurrences of each such event type. 199 "IP address" is used herein as a length independent term and includes 200 both IPv4 and IPv6 addresses. 202 2. Threats Considered 204 DNS cookies are intended to provide significant but limited 205 protection against certain attacks by off-path attackers as described 206 below. These attacks include denial-of-service, cache poisoning and 207 answer forgery. 209 2.1 Denial-of-Service Attacks 211 The typical form of the denial-of-service attacks considered herein 212 is to send DNS requests with forged source IP addresses to a server. 213 The intent can be to attack that server or some other selected host 214 as described below. 216 2.1.1 DNS Amplification Attacks 218 A request with a forged IP address generally causes a response to be 219 sent to that forged IP address. Thus the forging of many such 220 requests with a particular source IP address can result in enough 221 traffic being sent to the forged IP address to interfere with service 222 to the host at the IP address. Furthermore, it is generally easy in 223 the DNS to create short requests that produce much longer responses, 224 thus amplifying the attack. 226 The DNS Cookies mechanism can severely limit the traffic 227 amplification obtained by attackers off path for the server and the 228 attacked host. Enforced DNS cookies would make it hard for an off 229 path attacker to cause any more than rate-limited short error 230 responses to be sent to a forged IP address so the attack would be 231 attenuated rather than amplified. DNS cookies make it more effective 232 to implement a rate limiting scheme for error responses from the 233 server. Such a scheme would further restrict selected host denial- 234 of-service traffic from that server. 236 2.1.2 DNS Server Denial-of-Service 238 DNS requests that are accepted cause work on the part of DNS servers. 239 This is particularly true for recursive servers that may issue one or 240 more requests and process the responses thereto, in order to 241 determine their response to the initial request. And the situation 242 can be even worse for recursive servers implementing DNSSEC 243 ([RFC4033] [RFC4034] [RFC4035]) because they may be induced to 244 perform burdensome cryptographic computations in attempts to verify 245 the authenticity of data they retrieve in trying to answer the 246 request. 248 The computational or communications burden caused by such requests 249 may not dependent on a forged IP source address, but the use of such 250 addresses makes 251 + the source of the requests causing the denial-of-service attack 252 harder to find and 253 + restriction of the IP addresses from which such requests should 254 be honored hard or impossible to specify or verify. 256 Use of DNS cookies should enables a server to reject forged queries 257 from an off path attacker with relative ease and before any recursive 258 queries or public key cryptographic operations are performed. 260 2.2 Cache Poisoning and Answer Forgery Attacks 262 The form of the cache poisoning attacks considered is to send forged 263 replies to a resolver. Modern network speeds for well-connected hosts 264 are such that, by forging replies from the IP addresses of a DNS 265 server to a resolver for common names or names that resolver has been 266 induced to resolve, there can be an unacceptably high probability of 267 randomly coming up with a reply that will be accepted and cause false 268 DNS information to be cached by that resolver (the Dan Kaminsky 269 attack). This can be used to facilitate phishing attacks and other 270 diversion of legitimate traffic to a compromised or malicious host 271 such as a web server. 273 With the use of DNS cookies, a resolver can generally reject such 274 forged replies. 276 3. Comments on Existing DNS Security 278 Two forms of security have been added to DNS, data security and 279 message/transaction security. 281 3.1 Existing DNS Data Security 283 DNS data security is one part of DNSSEC and is described in 284 [RFC4033], [RFC4034], and [RFC4035] and updates thereto. It provides 285 data origin authentication and authenticated denial of existence. 286 DNSSEC is being deployed and can provide strong protection against 287 forged data; however, it has the unintended effect of making some 288 denial-of-service attacks worse because of the cryptographic 289 computational load it can require and the increased size in DNS 290 response packets that it tends to produce. 292 3.2 DNS Message/Transaction Security 294 The second form of security that has been added to DNS provides 295 "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931]. 296 TSIG could provide strong protection against the attacks for which 297 the DNS Cookies mechanism provide weak protection; however, TSIG is 298 non-trivial to deploy in the general Internet because of the burden 299 it imposes of pre-agreement and key distribution between client- 300 server pairs, the burden of server side key state, and because it 301 requires time synchronization between client and server. 303 TKEY [RFC2930] can solve the problem of key distribution for TSIG but 304 some modes of TKEY impose a substantial cryptographic computation 305 loads and can be dependent on the deployment of DNS data security 306 (see Section 3.1). 308 SIG(0) [RFC2931] provides less denial of service protection than TSIG 309 or, in one way, even DNS cookies, because it does not authenticate 310 requests, only complete transactions. In any case, it also depends 311 on the deployment of DNS data security and requires computationally 312 burdensome public key cryptographic operations. 314 3.3 Conclusions on Existing DNS Security 316 The existing DNS security mechanisms do not provide the services 317 provided by the DNS Cookies mechanism: lightweight message 318 authentication of DNS requests and responses with no requirement for 319 pre-configuration or per client server side state. 321 4. The COOKIE OPT Option 323 COOKIE is an OPT RR [RFC6891] option that can be included in the 324 RDATA portion of an OPT RR in DNS requests and responses. The option 325 length varies depending on the circumstance in which it is being 326 used. There are two cases as described below. Both use the same 327 OPTION-CODE; they are distinguished by their length. 329 The COOKIE OPT describced in this Section is used in the protocol 330 described in Section 5. An alternative COOKIE OPT is described in 331 Section 6 that is used in an alternative protocol described in 332 Section 7. 334 In a request sent by a client to a server when the client does not 335 know the server cookie, its length is 10, consisting of a 2 bytes DNS 336 error code field followed by the 8 byte Client Cookie as shown in 337 Figure 1. 339 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 340 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 341 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 342 | OPTION-CODE = {TBD} | OPTION-LENGTH = 10 | 343 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 344 | Error Code (2 bytes) | 345 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 346 | | 347 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 348 | | 349 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 351 Figure 1. COOKIE Option, Unknown Server Cookie 353 In a request sent by a client when a server cookie is known and in 354 all responses, the length is variable from 18 to 42 bytes, consisting 355 of a 2 byte DNS error field followed by the 8 bytes Client Cookie and 356 then the variable 8 to 32 bytes Server Cookie as shown in Figure 2. 357 The variability of the option length stems from the variable length 358 Server Cookie. The Server Cookie is an integer number of bytes with a 359 minimum size of 64 bits for security and a maximum size of 256 bits 360 for implementation convenience. 362 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 363 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 364 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 365 | OPTION-CODE = {TBD} | OPTION-LENGTH >= 18, <= 42 | 366 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 367 | Error Code (2 bytes) | 368 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 369 | | 370 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 371 | | 372 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 373 | | 374 / Server Cookie (variable size, 8 to 32 bytes) / 375 / / 376 +-+-+-+-... 378 Figure 2. COOKIE Option, Known Server Cookie 380 4.1 Client Cookie 382 The Client Cookie SHOULD be a pseudo-random function of the server IP 383 address and a secret quantity known only to the client. This client 384 secret SHOULD have at least 64 bits of entropy [RFC4086] and be 385 changed periodically (see Section 5.4). The selection of the pseudo- 386 random function is a matter private to the client as only the client 387 needs to recognize its own DNS cookies. 389 For further discussion of the Client Cookie field, see Section 5.1. 390 For example methods of determining a Client Cookie, see Appendix A. 392 A client MUST NOT use the same Client Cookie value for queries to all 393 servers. 395 4.2 Server Cookie 397 The Server Cookie SHOULD consist of or include a 64-bit or larger 398 pseudo-random function of the request source IP address, the request 399 Client Cookie, and a secret quantity known only to the server. (See 400 Section 8 for a discussion of why the Client Cookie is used as input 401 to the Server Cookie but the Server Cookie is not used as an input to 402 the Client Cookie.) This server secret SHOULD have at least 64 bits 403 of entropy [RFC4086] and be changed periodically (see Section 5.4). 404 The selection of the pseudo-random function is a matter private to 405 the server as only the server needs to recognize its own DNS cookies. 407 For further discussion of the Server Cookie field see Section 5.2. 408 For example methods of determining a Server Cookie, see Appendix B. 410 A server MUST NOT use the same Server Cookie value for responses to 411 all clients. 413 4.3 Error Code 415 In requests, the Error Code field MUST be zero and is ignored on 416 receipt. Replies have a COOKIE OPT with an Error Code equal to one of 417 the following four values: Zero (if the request they respond to had a 418 COOKIE OPT with a correct Server Cookie), NOCOOKIE, MFCOOKIE, or 419 BADCOOKIE. 421 NOCOOKIE and MFCOOKIE indicate that the server did not receive a 422 Client Cookie, either because there was no COOKIE OPT option in the 423 request (NOCOOKIE) or one was present but the COOKIE OPT option was 424 malformed as not being a valid length (MFCOOKIE). BADCOOKIE indicates 425 that the server did receive a Client Cookie but did not receive the 426 correct Server Cookie either because there was no Server Cookie 427 present or because it was not a valid value. 429 A server may choose to normally process a request, for example 430 returning the normal answer information for a QUERY, notwithstanding 431 a cookie error condition. For more information on error processing, 432 see Section 5. 434 5. DNS Cookies Protocol Description 436 This section discusses using DNS Cookies in the DNS Protocol. An 437 alternative protocol description appears in Section 7. 439 5.1 Originating Requests 441 A DNS client that implements DNS cookies includes one DNS Cookie 442 option in every DNS request it sends unless DNS cookies are disabled. 443 The COOKIE OPT option in a request always includes a zero Error Code 444 field and a Client Cookie as discussed in Section 4.1. 446 If the client has no Server Cookie obtained from a previous DNS 447 response and cached under the server's IP address, it uses the 448 shorter form of COOKIE OPT shown in Figure 1. If the client does have 449 such a cached Server Cookie, it uses the form of COOKIE OPT shown in 450 Figure 2 and also includes that cached Server Cookie in the DNS 451 option it sends. 453 5.2 Responding to Requests 455 The Server Cookie, when it occurs in a COOKIE OPT option in a 456 request, is intended to weakly assure the server that the request 457 came from a client that is both at the source IP address of the 458 request and using the Client Cookie included in the option. This weak 459 assurance is provided by the Server Cookie that server would send to 460 that client in an earlier response appearing as the Server Cookie 461 field in the request. 463 At a server where DNS Cookies are not implemented and enabled, the 464 presence of a COOKIE OPT option is ignored and the server responds as 465 before. 467 When DNS Cookies are implemented and enabled, there are four 468 possibilities: (1) there is no OPT RR at all in the request; (2) 469 there is no valid Client Cookie in the request because the COOKIE OPT 470 option in absent from the request or one is present but not a legal 471 length; (3) there is a valid length cookie option in the request with 472 no Server Cookie or an incorrect Server Cookie; or (4) there is a 473 cookie option in the request with a correct Server Cookie. The four 474 possibilities are discussed in the subsections below. 476 In the case of multiple COOKIE OPT options in a request, only the 477 first (the one closest to the DNS header) is considered. All others 478 are ignored. 480 5.2.1 No OPT RR 482 If there is no OPT RR in the request, the client does not support 483 EDNS since [RFC6891] requires that an OPT RR be included in a request 484 if the requester supports that feature. Under these circumstances, 485 the server cannot expect to ever receive a correct COOKIE OPT option 486 from the client as in Section 5.2.4. 488 The situation and server options available are the same as those in 489 Section 5.2.2 except that no OPT RR can be included in any response. 491 5.2.2 No Valid Client Cookie 493 A request with an OPT RR but no COOKIE OPT option or with a COOKIE 494 that is not a valid length (10 or 18 through 42) could be from a 495 client that does not implement DNS cookies or on which they are 496 disabled or it could be some form of abuse or broken client 497 implementation. A server on which DNS cookies are enabled has the 498 following three choices in responding to such a request: 500 (1) Silently discard the request. 502 (2) Not process the request other than returning a minimal length 503 error response. Because of the absence of a validly formatted 504 COOKIE OPT option in the request, it cannot be assumed that the 505 client would understand any new RCODE values. An RCODE of Refused 506 is returned and the Error Field of the returned COOKIE OPT option 507 is set to NOCOOKIE if there was no COOKIE OPT option in the 508 request and set to MFCOOKIE if such an option was present but not 509 a valid length. 511 (3) Process the request normally and provide a normal response except 512 that a COOKIE OPT option with a non-zero Error Field is included 513 as in point 2 above. The RCODE in the DNS Header is zero unless 514 some non-cookie error occurs in processing the request. 516 Server policy determines how often the server selects each of the 517 above response choices; however, if the request was received over 518 TCP, the server may wish to take the weak authentication provided by 519 the use of TCP into account, increasing the probability of choice 3 520 and decreasing the probability of choice 1 perhaps to the extent of 521 never choosing 1. For both response choices 2 and 3, the server 522 should consider setting TC=1 in the response so that future requests 523 from the client are more likely to be received with the weak 524 authentication that can be provided by TCP. 526 5.2.3 Bad or Absent Server Cookie 528 If a request is received with the COOKIE OPT option having no Server 529 Cookie value (length 10) or a bad Server Cookie value (length 18 to 530 42), it could be some attempted abuse or it could just be that the 531 client does not know a currently valid Server Cookie for the server 532 to which the request was sent. For example, the client might have an 533 old, no longer recognized Server Cookie 535 Servers MUST, at least occasionally, respond to such requests to 536 inform the client of the correct Server Cookie. This is necessary so 537 that such a client can bootstrap to the weakly secure state where 538 requests and responses have recognized Server Cookies and Client 539 Cookies. 541 In responding to such a request, the server has the following three 542 choices: 544 (1) Silently discard the request. 546 (2) Not process the request other than returning a minimal length 547 error response. Because of the correct length COOKIE OPT option 548 in the request, the client can be assume to understand the new 549 error codes assigned in this document. Both the Error Field in 550 the returned COOKIE OPT option and the extended RCODE are set to 551 BADCOOKIE. 553 (3) Processes the request normally and sends its usual response 554 including a COOKIE OPT option with an Error field of BADCOOKIE 555 and a zero RCODE (unless there was also a non-cookie error in 556 processing the request). 558 Server policy determines how often the server selects each of the 559 above response choices; however, if the request was received over 560 TCP, the server may wish to take the weak authentication provided by 561 the use of TCP into account, increasing the probability of choice 3 562 and decreasing the probability of choice 1 perhaps to the extent of 563 never choosing 1. 565 5.2.4 A Correct Server Cookie 567 If a server with enabled DNS cookies receives a request where the 568 COOKIE OPT option has a valid length and correct Server Cookie, it 569 processes the request normally and includes a COOKIE OPT option with 570 a zero Error Field in the response. Such a response might have a 571 non-zero RCODE if a non-cookie error occurs in processing the 572 request. 574 5.3 Processing Responses 576 The Client Cookie, when it occurs in a COOKIE OPT option in a DNS 577 reply, is intended to weakly assure the client that the reply came 578 from a server at the source IP address use in the response packet 579 because the Client Cookie value is the value that client would send 580 to that server in a request. If there are multiple COOKIE OPT options 581 in a DNS reply, all but the first (the one closest to the DNS Header) 582 are ignored. 584 A DNS client where DNS cookies are implemented and enabled examines 585 response for DNS cookies and MUST discard the response if it contains 586 an illegal COOKIE OPT option length or an incorrect Client Cookie 587 value. If the COOKIE OPT option Client Cookie is correct, the client 588 caches the Server Cookie provided even if the response is an error 589 response (RCODE non-zero). 591 If the reply extended RCDOE is BADCOOKIE, it means that the server 592 was unwilling to process the request because it did not have the 593 correct Server Cookie in it. The client should retry the request 594 using the new Server Cookie from the response. 596 If the RCODE is some value other than BADCOOKIE, including zero, the 597 response is then processed normally. 599 5.4 Client and Server Secret Rollover 601 Clients and servers MUST NOT continue to use the same secret in new 602 queries and responses, respectively, for more than 14 days and SHOULD 603 NOT continue to do so for more than 1 day. Many clients rolling over 604 their secret at the same time could briefly increase server traffic 605 and exactly predictable rollover times for clients or servers might 606 facilitate guessing attacks. For example, an attacker might increase 607 the priority of attacking secrets they believe will be in effect for 608 an extended period of time. To avoid rollover synchronization and 609 predictability, it is RECOMMENDED that pseudorandom jitter of at 610 least 30% be applied to the time of a scheduled rollover of a DNS 611 cookie secret. 613 It is RECOMMENDED that a client keep the Client Cookie it is 614 expecting in a reply associated with the outstanding query to avoid 615 rejection of replies due to a bad Client Cookie right after a change 616 in the Client Secret. It is RECOMMENDED that a server retain its 617 previous secret for a period of time not less than 1 second or more 618 than 3 minutes, after a change in its secret, and consider queries 619 with Server Cookies based on its previous secret to have a correct 620 Server Cookie during that time. 622 Receiving a sudden increased level of requests with bad Server 623 Cookies or replies with bad Client Cookies would be a good reason to 624 believe a server or client is likely to be under attack and should 625 consider more frequent rollover of its secret. 627 5.5 Implementation Requirement 629 DNS clients and servers SHOULD implement DNS cookies to decrease 630 their vulnerability to the threats discussed in Section 2. 632 6. Simple DNS Cookie Option 634 The Simple DNS Cookie Option is a alternative DNS COOKIE option 635 format that is implemented in BIND 9.10 using a experimental option 636 code. It differs from the the COOKIE OPT Option (Section 4) in that 637 it does not contain a error code and as a consequence there is 638 mimimal error handling required. Only one of COOKIE OPT or Simple 639 DNS Cookie Option will be in the final document. Both are present in 640 this document for comparision. 642 The Simple DNS Cookie Option is a OPT RR [RFC6891] option that can be 643 included in the RDATA portion of an OPT RR in DNS requests and 644 responses. The option length varies depending on the circumstances 645 in which it is being used. There are two case as described below. 646 Both use the same OPTION-CODE; they are distinguished by there 647 length. 649 In a request sent by a client to a server when the client does not 650 know the server cookie, its length is 8, consisting a 8 byte Client 651 Cookie as shown in Figure 3. 653 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 654 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 655 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 656 | OPTION-CODE = {TBD} | OPTION-LENGTH = 8 | 657 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 658 | | 659 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 660 | | 661 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 663 Figure 3. Simple COOKIE Option, Unknown Server Cookie 665 In a request sent by a client when a server cookie is known and in 666 all responses, the length is variable from 16 to 40 bytes, consisting 667 of a 8 bytes Client Cookie followed by the variable 8 to 32 bytes 668 Server Cookie as shown in Figure 4. The variability of the option 669 length stems from the variable length Server Cookie. The Server 670 Cookie is an integer number of bytes with a minimum size of 64 bits 671 for security and a maximum size of 256 bits for implementation 672 convenience. 674 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 675 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 676 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 677 | OPTION-CODE = {TBD} | OPTION-LENGTH >= 16, <= 40 | 678 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 679 | | 680 +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ 681 | | 682 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 683 | | 684 / Server Cookie (variable size, 8 to 32 bytes) / 685 / / 686 +-+-+-+-... 688 Figure 4. Simple COOKIE Option, Known Server Cookie 690 6.1 Simple Client Cookie 692 [This section is identical to section 4.1] 694 The Client Cookie SHOULD be a pseudo-random function of the server IP 695 address and a secret quantity known only to the client. This client 696 secret SHOULD have at least 64 bits of entropy [RFC4086] and be 697 changed periodically (see Section 5.4). The selection of the pseudo- 698 random function is a matter private to the client as only the client 699 needs to recognize its own DNS cookies. 701 For further discussion of the Client Cookie field, see Section 5.1. 702 For example methods of determining a Client Cookie, see Appendix A. 704 A client MUST NOT use the same Client Cookie value for queries to all 705 servers. 707 6.2 Simple Server Cookie 709 [This section is identical to section 4.2] 711 The Simple Server Cookie SHOULD consist of or include a 64-bit or 712 larger pseudo-random function of the request source IP address, the 713 request Simple Client Cookie, and a secret quantity known only to the 714 server. (See Section 8 for a discussion of why the Simple Client 715 Cookie is used as input to the Simple Server Cookie but the Simple 716 Server Cookie is not used as an input to the Simple Client Cookie.) 717 This server secret SHOULD have at least 64 bits of entropy [RFC4086] 718 and be changed periodically (see Section 5.4). The selection of the 719 pseudo-random function is a matter private to the server as only the 720 server needs to recognize its own DNS cookies. 722 For further discussion of the Simple Server Cookie field see Section 723 5.2. For example methods of determining a Server Cookie, see 724 Appendix B. 726 A server MUST NOT use the same Server Cookie value for responses to 727 all clients. 729 7. Simple DNS Cookies Protocol Description 731 This section discusses using Simple DNS Cookies in the DNS Protocol. 733 7.1 Originating Requests (Simple) 735 A DNS client that implements DNS includes one DNS Cookie option in 736 every DNS requests it sends unless DNS cookies are disabled. 738 If the client has a cached server cookie for the server against its 739 IP address it includes that in the option along with the client 740 cookie (Figure 4) otherwise it just sends a option with a client 741 cookie (Figure 3). 743 7.2 Responding to Request (Simple) 745 The Server Cookie, when included in a COOKIE option in a request, is 746 intended to weakly assure that server that the request has come from 747 a client that it has responsed to in the past and is both at the same 748 source address and is using the same Client Cookie in the option. 750 At a server where Simple DNS Cookies are not implemented and enabled, 751 presence of a COOKIE OPT option is ignored and the server responds as 752 before. 754 When DNS Cookies are implemented and enabled, there are four 755 possibilities: (1) there is no OPT RR at all in the request; (2) 756 there is no valid Client Cookie in the request because the COOKIE OPT 757 option in absent from the request or one is present but not a legal 758 length; (3) there is a valid length cookie option in the request with 759 no Server Cookie or an incorrect Server Cookie; or (4) there is a 760 cookie option in the request with a correct Server Cookie. The four 761 possibilities are discussed in the subsections below. 763 In the case of multiple COOKIE OPT options in a request, only the 764 first (the one closest to the DNS header) is considered. All others 765 are ignored. 767 7.2.1 No Opt RR or No COOKIE OPT option 769 If there is no OPT record or on COOKIE OPT option present in the 770 request then the server responds to the request as if it doesn't 771 understand the COOKIE OPT. 773 7.2.2 Malformed COOKIE OPT option 775 If the COOKIE OPT is too short to contain a Client Cookie then 776 FORMERR is generated. If the COOKIE OPT is longer than that required 777 to hold a COOKIE OPT with just a Client Cookie (8) but is shorter 778 that the mimimum COOKIE OPT with both both a Client and Server Cookie 779 (16) then FORMERR is generated. If the COOKIE OPT is longer than the 780 maximum valid COOKIE OPT (40) then a FORMERR is generated. 782 In summary valid cookie lengths are 8 and 16 to 40 inclusive. 784 7.2.3 Only a CLIENT Cookie 786 The server SHALL process the query as if the Client Cookie was not 787 present. In addition it SHALL generate its own COOKIE OPT containing 788 both the client cookie copied from the request and a server cookie it 789 has generated and adds this COOKIE OPT to the response's OPT record. 791 7.2.4 A Client Cookie and Server Cookie 793 The server shall examine the Server Cookie to determine if it is a 794 valid server cookie it has generated. This examination will result 795 in a deterimination of whether the server cookie is valid or not. 797 7.2.4.1 A Client Cookie and Invalid Server Cookie 799 This can occur due to a stale server cookie being returned, a clients 800 IP address changing without the DNS client being aware, or a attempt 801 to spoof the client. 803 The server SHALL process the query as if the Client Cookie was not 804 present. In addition it SHALL generate its own COOKIE OPT containing 805 both the client cookie copied from the request and a valid server 806 cookie it has generated and adds this COOKIE OPT to the response's 807 OPT record. 809 7.2.4.2 A Client Cookie and Valid Server Cookie 811 When this occurs the server can assume that it is talking to a client 812 that it has talked to before and defensive measures for spoofed UDP 813 queries, if any, are no longer required. 815 The server SHALL process the query and include a COOKIE OPT in the 816 response by (a) copying the complete COOKIE OPT from the request or 817 (b) generating a new COOKIE OPT containing both the client cookie 818 copied from the request and a valid server cookie it has generated. 820 8. NAT Considerations and AnyCast Server Considerations 822 In the Classic Internet, DNS Cookies could simply be a pseudo-random 823 function of the client IP address and a sever secret or the server IP 824 address and a client secret. You would want to compute the Server 825 Cookie that way, so a client could cache its Server Cookie for a 826 particular server for an indefinitely amount of time and the server 827 could easily regenerate and check it. You could consider the Client 828 Cookie to be a weak client signature over the server IP address that 829 the client checks in replies and you could extend this weak signature 830 to cover the request ID, for example, or any other information that 831 is returned unchanged in the reply. 833 But we have this reality called NAT [RFC3022], Network Address 834 Translation (including, for the purposes of this document, NAT-PT, 835 Network Address and Protocol Translation, which has been declared 836 Historic [RFC4966]). There is no problem with DNS transactions 837 between clients and servers behind a NAT box using local IP 838 addresses. Nor is there a problem with NAT translation of internal 839 addresses to external addresses or translations between IPv4 and IPv6 840 addresses, as long as the address mapping is relatively stable. 841 Should the external IP address an internal client is being mapped to 842 change occasionally, the disruption is little more than when a client 843 rolls-over its DNS COOKIE secret. And normally external access to a 844 DNS server behind a NAT box is handled by a fixed mapping which 845 forwards externally received DNS requests to a specific host. 847 However, NAT devices sometimes also map ports. This can cause 848 multiple DNS requests and responses from multiple internal hosts to 849 be mapped to a smaller number of external IP addresses, such as one 850 address. Thus there could be many clients behind a NAT box that 851 appear to come from the same source IP address to a server outside 852 that NAT box. If one of these were an attacker (think Zombie or 853 Botnet), that behind-NAT attacker could get the Server Cookie for 854 some server for the outgoing IP address by just making some random 855 request to that server. It could then include that Server Cookie in 856 the COOKIE OPT of requests to the server with the forged local IP 857 address of some other host and/or client behind the NAT box. 858 (Attacker possession of this Server Cookie will not help in forging 859 responses to cause cache poisoning as such responses are protected by 860 the required Client Cookie.) 862 To fix this potential defect, it is necessary to distinguish 863 different clients behind a NAT box from the point of view of the 864 server. It is for this reason that the Server Cookie is specified as 865 a pseudo-random function of both the request source IP address and 866 the Client Cookie. From this inclusion of the Client Cookie in the 867 calculation of the Server Cookie, it follows that a stable Client 868 Cookie, for any particular server, is needed. If, for example, the 869 request ID was included in the calculation of the Client Cookie, it 870 would normally change with each request to a particular server. This 871 would mean that each request would have to be sent twice: first to 872 learn the new Server Cookie based on this new Client Cookie based on 873 the new ID and then again using this new Client Cookie to actually 874 get an answer. Thus the input to the Client Cookie computation must 875 be limited to the server IP address and one or more things that 876 change slowly such as the client secret. 878 In principle, there could be a similar problem for servers, not due 879 to NAT but due to mechanisms like anycast which may cause queries to 880 a DNS server at an IP address to be delivered to any one of several 881 machines. (External queries to a DNS server behind a NAT box usually 882 occur via port forwarding such that all such queries go to one host.) 883 However, it is impossible to solve this the way the similar problem 884 was solved for NATed clients; if the Server Cookie was included in 885 the calculation of the Client Cookie the same way the Client Cookie 886 is included in the Server Cookie, you would just get an almost 887 infinite series of errors as a request was repeatedly retried. 889 For servers accessed via anycast to successfully support DNS COOKIES, 890 the server clones must either all use the same server secret or the 891 mechanism that distributes queries to them must cause the queries 892 from a particular client to go to a particular server for a 893 sufficiently long period of time that extra queries due to changes in 894 Server Cookie resulting from accessing different server machines are 895 not unduly burdensome. (When such anycast-accessed servers act as 896 recursive servers or otherwise act as clients they normally use a 897 different unique address to source their queries to avoid confusion 898 in the delivery of responses.) 900 For simplicity, it is RECOMMENDED that the same server secret be used 901 by each DNS server in a set of anycast servers. If there is limited 902 time skew in updating this secret in different anycast servers, this 903 can be handled by a server accepting requests containing a Server 904 Cookie based on either its old or new secret for the maximum likely 905 time period of such time skew (see also Section 5.4). 907 9. Deployment 909 The DNS cookies mechanism is designed for incremental deployment and 910 to complement the orthogonal techniques in [RFC5452]. Either or both 911 techniques can be deployed independently at each DNS server and 912 client. 914 In particular, a DNS server or client that implements the DNS COOKIE 915 mechanism can interoperate successfully with a DNS client or server 916 that does not implement this mechanism although, of course, in this 917 case it will not get the benefit of the mechanism and the server 918 involved might choose to severely rate limit responses. When such a 919 server or client interoperates with a client or server which also 920 implements the DNS cookies mechanism, they get the weak security 921 benefits of the DNS Cookies mechanism. 923 10. IANA Considerations 925 IANA is requested to assign the following four code points: 927 The OPT option value for COOKIE is [10 suggested]. 929 [The following error codes are not required for the Simple COOKIE 930 OPT.] 932 Three new DNS error codes in the range above 16 and below 3,840 as 933 shown below: 935 RCODE Name Description Reference 936 -------- --------- ----------------- --------------- 937 TBD1[23] NOCOOKIE No client cookie. [this document] 938 TBD2[24] MFCOOKIE Malformed cookie. [this document] 939 TBD3[25] BADCOOKIE Bad/missing server cookie. [this document] 941 11. Security Considerations 943 DNS Cookies provide a weak form of authentication of DNS requests and 944 responses. In particular, they provide no protection against "on- 945 path" adversaries; that is, they provide no protection against any 946 adversary that can observe the plain text DNS traffic, such as an on- 947 path router, bridge, or any device on an on-path shared link (unless 948 the DNS traffic in question on that path is encrypted). 950 For example, if a host is connected via an unsecured IEEE 802.11 link 951 (Wi-Fi), any device in the vicinity that could receive and decode the 952 802.11 transmissions must be considered "on-path". On the other hand, 953 in a similar situation but one where 802.11 Robust Security (WPAv2) 954 is appropriately deployed on the Wi-Fi network nodes, only the Access 955 Point via which the host is connecting is "on-path" as far as the 956 802.11 link is concerned. 958 Despite these limitations, deployment of DNS Cookies on the global 959 Internet is expected to provide a substantial reduction in the 960 available launch points for the traffic amplification and denial of 961 service forgery attacks described in Section 2 above. 963 Should stronger message/transaction security be desired, it is 964 suggested that TSIG or SIG(0) security be used (see Section 3.2); 965 however, it may be useful to use DNS Cookies in conjunction with 966 these features. In particular, DNS Cookies could screen out many DNS 967 messages before the cryptographic computations of TSIG or SIG(0) are 968 required and, if SIG(0) is in use, DNS Cookies could usefully screen 969 out many requests given that SIG(0) does not screen requests but only 970 authenticates the response of complete transactions. 972 11.1 Cookie Algorithm Considerations 974 The cookie computation algorithm for use in DNS Cookies SHOULD be 975 based on a pseudo-random function at least as strong as [FNV] because 976 an excessively weak or trivial algorithm could enable adversaries to 977 guess cookies. However, in light of the weak plain-text token 978 security provided by DNS Cookies, a strong cryptography hash 979 algorithm may not be warranted in many cases, and would cause an 980 increased computational burden. Nevertheless there is nothing wrong 981 with using something stronger, for example, HMAC-SHA256-64 [RFC6234], 982 assuming a DNS processor has adequate computational resources 983 available. DNS processors that feel the need for somewhat stronger 984 security without a significant increase in computational load should 985 consider more frequent changes in their client and/or server secret; 986 however, this does require more frequent generation of a 987 cryptographically strong random number [RFC4086]. See Appendices A 988 and B for specific examples of cookie computation algorithms. 990 Normative References 992 [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate 993 Requirement Levels", BCP 14, RFC 2119, March 1997. 995 [RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker, 996 "Randomness Requirements for Security", BCP 106, RFC 4086, June 997 2005. 999 [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 1000 for DNS (EDNS(0))", STD 75, RFC 6891, April 2013. 1002 Informative References 1004 [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non- 1005 Cryptographic Hash Algorithm", draft-eastlake-fnv, work in 1006 progress. 1008 [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 1009 Wellington, "Secret Key Transaction Authentication for DNS 1010 (TSIG)", RFC 2845, May 2000. 1012 [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY 1013 RR)", RFC 2930, September 2000. 1015 [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures 1016 ( SIG(0)s )", RFC 2931, September 2000. 1018 [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network 1019 Address Translator (Traditional NAT)", RFC 3022, January 2001. 1021 [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 1022 Rose, "DNS Security Introduction and Requirements", RFC 4033, 1023 March 2005. 1025 [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 1026 Rose, "Resource Records for the DNS Security Extensions", RFC 1027 4034, March 2005. 1029 [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. 1030 Rose, "Protocol Modifications for the DNS Security Extensions", 1031 RFC 4035, March 2005. 1033 [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network 1034 Address Translator - Protocol Translator (NAT-PT) to Historic 1035 Status", RFC 4966, July 2007. 1037 [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More 1038 Resilient against Forged Answers", RFC 5452, January 2009. 1040 [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash 1041 Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May 1042 2011. 1044 Acknowledgements 1046 The contributions of the following are gratefully acknowledged: 1048 Tim Wicinski 1050 Appendix A: Example Client Cookie Algorithms 1052 A.1 A Simple Algorithm 1054 An simple example method to compute Client Cookies is the FNV-64 1055 [FNV] of the server IP address and the client secret. That is 1057 Client Cookie = FNV-64 ( Client Secret | Server IP Address ) 1059 where "|" indicates concatenation. 1061 A.2 A More Complex Algorithm 1063 A more complex algorithm to calculate Client Cookies is give below. 1064 It uses more computational resources than the simpler algorithm shown 1065 in A.1. 1067 Client Cookie = HMAC-SHA256-64 ( Client Secret, Server IP Address 1068 ) 1070 Appendix B: Example Server Cookie Algorithms 1072 B.1 A Simple Algorithm 1074 An example simple method producing a 64-bit Server Cookie is the 1075 FNV-64 [FNV] of the request IP address, the Client Cookie, and the 1076 server secret. That is 1078 Server Cookie = 1079 FNV-64 ( Server Secret | Request IP Address | Client Cookie ) 1081 where "|" represents concatenation. 1083 B.2 A More Complex Algorithm 1085 Since the Server Cookie is variable size, the server can store 1086 various information in that field as long as it is hard for an 1087 adversary to guess the entire quantity used for weak authentication. 1088 There should be 64 bits of entropy in the Server Cookie; for example 1089 it could have a sub-field of 64-bits computed pseudo-randomly with 1090 the server secret as one of the inputs to the pseudo-random function. 1091 Types of additional information that could be stored include a time 1092 stamp and/or a nonce. 1094 The example below is one variation for the Server Cookie that has 1095 been implemented in a beta release of BIND where the Server Cookie is 1096 128 bits composed as follows: 1098 Sub-field Size 1099 --------- --------- 1100 Nonce 32 bits 1101 Time 32 bits 1102 Hash 64 bits 1104 With this algorithm, the server sends a new 128-bit cookie back with 1105 every request. The Nonce field assures a low probability that there 1106 would be a duplicate. 1108 The Time field gives the server time and makes it easy to reject old 1109 cookies. 1111 The Hash part of the Server Cookie is the hard-to-guess part. In the 1112 beta release of BIND, its computation can be configured to use AES, 1113 HMAC-SHA1, or, as shown below, HMAC-SHA256: 1115 hash = 1116 HMAC-SHA256-64 ( Server Secret, 1117 (Client Cookie | nonce | time | client IP Address) ) 1119 where "|" represents concatenation. 1121 Author's Address 1123 Donald E. Eastlake 3rd 1124 Huawei Technologies 1125 155 Beaver Street 1126 Milford, MA 01757 USA 1128 Telephone: +1-508-333-2270 1129 EMail: d3e3e3@gmail.com 1131 Mark Andrews 1132 Internet Systems Consortium 1133 950 Charter Street 1134 Redwood City, CA 94063 USA 1136 Email: marka@isc.org 1138 Copyright, Disclaimer, and Additional IPR Provisions 1140 Copyright (c) 2015 IETF Trust and the persons identified as the 1141 document authors. All rights reserved. 1143 This document is subject to BCP 78 and the IETF Trust's Legal 1144 Provisions Relating to IETF Documents 1145 (http://trustee.ietf.org/license-info) in effect on the date of 1146 publication of this document. Please review these documents 1147 carefully, as they describe your rights and restrictions with respect 1148 to this document. Code Components extracted from this document must 1149 include Simplified BSD License text as described in Section 4.e of 1150 the Trust Legal Provisions and are provided without warranty as 1151 described in the Simplified BSD License.